In the manufacture of semiconductor devices, ion implantation systems are employed to dope a semiconductor wafer or other workpiece with impurities. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam. The ion beam is typically mass analyzed to select ions of a desired charge-to-mass ratio and then directed at the surface of a semiconductor wafer in order to implant the wafer with the dopant element. The ions of the beam penetrate the surface of the wafer to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. A typical ion implanter includes an ion source for generating the ion beam, a beamline assembly including a mass analysis apparatus for mass resolving the ion beam using magnetic fields, and a target chamber containing the semiconductor wafer or workpiece to be implanted by the ion beam.
Typically, the ions generated from the ion source are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may further include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating the ion implanter, this passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
The mass of an ion relative to the charge thereon (i.e., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam that reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway that will effectively separate ions of different charge-to-mass ratios.
The ion beam is focused and directed at a desired surface region of the workpiece. Typically, the energetic ions of the ion beam are accelerated to a predetermined energy level to penetrate into the bulk of the workpiece. The ions, for example, are embedded into the crystalline lattice of the material to form a region of desired conductivity, with the energy of the ion beam generally determining the depth of implantation. Examples of ion implantation systems include those available from Axcelis Technologies of Beverly, Mass.
Operation of a typical ion implanter or other ion beam equipment (e.g., a linear accelerator), however, may result in the production of contaminant particles. The contaminant particles, for example, may be less than about 1 μm in size. The momentum of the ions in the beam that strike the particles, in turn, cause the particles to be transported with the beam, although typically at a speed much less than the ions. Consequently, particles entrained in an ion beam may be transported with the beam toward the workpiece, thus resulting in undesired contamination at the wafer.
Therefore, it is desirable to increase the utilization rate of the ion beam, therein increasing efficiency of the system, as well as mitigating potential workpiece contamination.